The invention relates generally to the field of photovoltaic (PV) solar power systems, and more specifically to circuits for protecting PV active bypass circuits from damage caused by electrical surges.
FIG. 1 is a high level block diagram of a conventional PV solar power system 10 including a plurality of PV subsections 11 connected in series. Each PV subsection 11 comprises a plurality of PV cells that are serially connected between a positive terminal 12 and a negative terminal 13. For example, a typical PV subsection includes twenty four PV cells, and produces about 12V between 12 and 13 in full sunlight. An inverter 15 converts the dc voltage to ac and has an output 16 for coupling to the electrical power grid. There is also usually a disconnect switch 17 for shutting down the system 10.
Since the PV subsections 11 are connected in series, the current is the same in each subsection. Therefore, when one subsection is shaded (e.g., by a tree branch, or chimney) it acts like a bottleneck, restricting current flow in the entire string. The unshaded PV subsections try to force current flow through the shaded subsection, resulting in the shaded subsection becoming reverse-biased. But a reverse-biased PV cell dissipates energy instead of producing energy, so the shaded subsection gets hot, and can even be permanently damaged. The well known remedy is to include bypass diodes 14 that allow current to flow around the shaded PV subsections, rather than through them. Thus, the bypass diodes 14 protect the PV subsections from damage due to reverse bias, and also avoid a serious reduction in system 10 efficiency when the string is partially shaded.
A common problem in PV systems, such as 10, is overheating in one or more of the bypass diodes 14. One solution is to replace the conventional bypass diodes 14 with active bypass circuits. There are many examples of such active bypass circuits in the prior art such as: U.S. Patent Application Publication number 2010/0002349 (La Scala, et al), U.S. Pat. No. 7,898,114 (Schmidt, et al), U.S. Patent Application Publication number 2009/0014050 (Haaf), and U.S. Patent Application Publication number 2011/0006232 (Fahrenbruch, et al).
FIG. 2 is a high level block diagram that is typical of such prior art, showing an active bypass circuit 20 comprising: a metal-oxide-semiconductor field-effect transistor (MOSFET) 22 with an integral body diode 21, and a power-supply/control circuit 24. When the PV subsection 11 is partially shaded, the string current initially flows through the MOSFET's body diode 21, creating a voltage (VDS) of approximately −500 mV from drain 12 to source 13. The power-supply/control circuit 24 amplifies VDS, producing approximately 5V between the MOSFET's gate 23 and it's source 13, thereby turning on the MOSFET 22 and reducing heat dissipation. When the PV subsection 11 is unshaded, the polarity of the drain-to-source voltage reverses, causing the power-supply/control circuit 24 to shut down and discharge the gate-to-source capacitance of the MOSFET 22, thereby turning off the MOSFET 22 again.
Another problem with conventional bypass diodes 14 is low reliability. For example, a 2012 report (Kato, et at) from Japan's Research Center for Photovoltaic Technologies (RCPVT) found that 47% of the 1272 solar power modules they examined, at a large PV installation called Mega-Solartown, had at least one failed bypass diode, after just eight years of service. And in 2010 an official report from the Solar American Board of Codes and Standards (www.solarabcs.org) stated “ . . . undetected bypass diode failures may be an endemic industry-wide sleeper problem . . . ”.
And yet, the PV industry still knows little about the true extent or causes of these bypass diode failures. One of the main suspected causes is electrical surges, which may destroy the diodes outright, or just weaken them, making them more susceptible to thermal runaway. There are at least two types of surges that can happen in PV systems: an inrush surge when the cutoff switch 17 is closed; and lightning-induced surges.
For example, FIG. 1 shows how a nearby lightning strike can induce current surges that damage or destroy bypass diodes 14. There are many places in the world where lightning strikes are frequent, and a lightning rod 6 is often placed in close proximity to a solar power array to prevent the lightning 5 from striking the array directly. The lightning discharge current IDIS—which can easily exceed 40 kA—flows down the lightning rod 6 and into earth 8 via a ground wire 7. An intense magnetic field is formed around the wire 7. If the ground wire 7 comes close to a PV subsection 11, as shown at the bottom of FIG. 1, then some of the magnetic flux lines 9 can link the circuit loop consisting of the PV subsection 11 and the bypass diode 14, causing an induced current ISURGE to flow through the bypass diode 14.
ISURGE can exceed 200 A at it's peak, and can flow in either direction. If ISURGE flows through the bypass diode 14 in the forward direction, there is only a small voltage drop across the diode, typically 2V or less. However, if ISURGE flows through the bypass diode 14 in the reverse direction, the diode 14 goes into avalanche breakdown, and the voltage across the diode 14 is typically about 50V. So, reverse current flow is the worst case by far, because the diode 14 absorbs much more energy.
For example, assume the peak surge current is 200 A, and the avalanche voltage is 50V. Then the peak power in the diode 14 during the surge is (200 A)(50V)=10 kW. But the surge typically has an effective width of only about 20 μs, so the energy absorbed by the diode is roughly (20 μs)(10 kW)=200 mJ. The diode must absorb all the energy because the surge happens too quickly for heat to diffuse out through the diode's package, so heat-sinks are of no use in reducing the sudden spike in junction temperature. Consequently, the diode's junction temperature suddenly shoots up, as much as 70° C., possibly with catastrophic results. In fact, some Schottky diodes used for bypass in PV systems fail at only about 50 mJ avalanche energy.
Active bypass circuits such as 20 can usually absorb more energy than traditional Schottky bypass diodes, but not much. For example, low-cost MOSFETs used in active bypass circuits for PV systems typically have avalanche energy ratings of about 75 mJ to 100 mJ.
Therefore, there is a need in the solar power industry for a low-cost means of protecting MOSFETs in active bypass circuits against damage caused by electrical surges.